This type of accident, known in the technical literature by the acronym CFIT, deriving from the expression “Controlled Flight Into Terrain”, was one of the main causes of transport aircraft accidents and of the number of people killed in recent years. However, thanks to concepts of signaling the risk of collision with the terrain and to the associated onboard systems known by the expression TAWS (Terrain Awareness & Alerting Systems), including in particular GCAS and T2CAS (developed and marketed by Thales) most of the accidents of this type are now avoided.
For more details on the concepts implemented in the TAWS systems, reference can profitably be made to the U.S. Pat. No. 5,488,563, U.S. Pat. No. 5,414,631, U.S. Pat. No. 5,638,282, U.S. Pat. No. 5,677,842, U.S. Pat. No. 6,088,654, U.S. Pat. No. 6,317,663, U.S. Pat. No. 6,480,120 and to the French patent applications FR 2.813.963, FR 2.842.594, FR 2.848661, FR 2.860.292, FR 2.864.270, FR 2.864.312, FR 2.867.851, FR 2.868.835.
The onboard systems for preventing risks of collision with the terrain that are currently in service are mostly based on a simple upward terrain avoidance maneuver of the so-called “Pull-Up” type, consisting of a full-throttle climb preceded by a leveling of the wings if the aircraft was turning and hereinafter called “standard avoidance maneuver” or “standard avoidance trajectory”.
However, there are situations of risk of collision with the terrain that cannot be resolved by a simple upward avoidance maneuver of the “Pull-up” type. For example, when a risk of collision with the ground occurs when the aircraft is turning along significant relief, even when the aircraft is directed in a straight line towards reliefs that are significantly higher, that cannot be crossed given the climb capabilities of the airplane. These situations are signaled by the most advanced TAWS systems (typically, the TAWS systems developed by Thales) by means of a specific “Avoid terrain” alarm that is different from the usual “Pull-up” alarm, but without indication of an avoidance trajectory making it possible to clear the dangerous situation. The crew, which is in a dangerous situation that it cannot get out of by a simple upward avoidance maneuver, must urgently decide, often in the absence of visibility and only with sight of the threatening reliefs appearing on the navigation screen, on a change of heading opening up once again the possibility of an upward avoidance of the relief.
The provision of an indication of the best avoidance trajectory or trajectories to be taken when there is a proven risk of collision with the terrain is therefore necessary to further improve the TAWS systems.
Moreover, since the crew can often be in situations where it is not fully aware of the imminence of the danger and where it delays the avoidance maneuver, it is highly desirable, over and above indications on the best avoidance trajectory or trajectories to be taken, for the avoidance trajectory to be able to be automated.
It should be noted that there are onboard certain military airplanes terrain following systems which automatically avoid the relief and the obstacles on the ground, but the operational objectives are fundamentally different from those of the TAWS systems intended for transport aircraft.
Indeed, the aim of the terrain following function is to keep a fighter airplane as long as possible close to the ground, at a predetermined height, in order to have it not seen by hostile forces. Each time that a risk of collision with the ground occurs, this function undertakes an avoidance maneuver of the smallest possible amplitude to remain glued to the terrain. In the case of transport aircraft, helicopters and even drones for certain missions, the operational objective is not to remain glued to the terrain but to follow trajectories programmed manually or automatically, via a flight management computer (FMS) or an automatic pilot PA, as much as possible avoiding any significant alteration of this trajectory, except to avoid the relief presenting proven risks of collision. For such aircraft, the initiation of a relief avoidance maneuver must be undertaken only in the presence of a proven risk, and as infrequently as possible in order to reduce the forces on the airframe and to respect the comfort of the passengers. Applying logics that are acceptable for fighter airplanes is absolutely not appropriate to the case of the transport aircraft considered here.
Also known, from the U.S. Pat. No. 5,892,462, is a TAWS system that uses a modeling of the behavior of the aircraft to generate a protection volume linked to the aircraft and to plot avoidance trajectories that are practicable for the aircraft. The protection volume surrounds an extrapolation of the current trajectory of the aircraft extended by a climb trajectory prediction. Its penetration by the terrain is likened to a risk of collision with the terrain. In case of proven risk of collision with the terrain, the system works back step by step through the current trajectory extrapolation to determine the limit point of the trajectory of the aircraft up to which a purely vertical avoidance maneuver can be successfully applied. When the limit point is passed or can no longer be determined, the system then initiates a search for a practicable lateral avoidance trajectory by an angular scanning of the terrain profiles, either side of the current trajectory of the aircraft.
This TAWS system gives a warning when the limit point for application of a purely vertical avoidance maneuver is passed and emits an alarm when it appears that it will no longer be possible to find a lateral avoidance trajectory. It addresses the concerns of a better assistance to a crew confronted with the need to execute a lateral avoidance maneuver because it is also capable of acting on the flight director to guide the pilot in the following of a lateral avoidance trajectory or on the automatic pilot to execute a lateral avoidance trajectory. This system, which does not appear to be operational at the present time, presents the drawback of using a modeling of the dynamic behavior of the aircraft which is complex to implement and poses major problems of certification with a view to operational implementation because the level of precision of this behavioral modeling affects the dependability of the TAWS system itself. In its search for the first possible way out from a short-term risk of collision with the terrain, it also has the drawback of not being concerned with minimizing the alterations made to the initial trajectory and the medium-term risks of collision with the terrain. Moreover, this search is initiated only after the limit point for a purely vertical avoidance maneuver has been passed, which limits its application to situations where the airplane is normally behind this limit point without running into any danger with the relief, for example in the case of a turn at low height close to high reliefs.
The applicant has also proposed, in a French patent application filed under the number 05 11457 on 10 Nov. 2005, a terrain avoidance system for transport aircraft that accompanies the alarms of “Avoid terrain” type indicating to the pilot that he has passed the limit point for success of a standard vertical avoidance maneuver, or complementary indications such as the azimuth sectors suitable for a clearance maneuver or a predefined clearance trajectory, or an automatic engagement of a predefined avoidance trajectory. This system implements, for the determination of the limit point, the monitoring of a penetration of a model of the terrain being flown over in a first protection volume linked to the aircraft and configured to model a standard vertical avoidance maneuver trajectory executed without delay, said trajectory being advantageously predefined according to parameters such as the type of the aircraft, its weight and even its inertia or climb delay, and for the determination of the clearance azimuth sectors, an analysis of the penetrations of the model of terrain being flown over in a second protection volume with large azimuth aperture, linked to the aircraft and configured to contain the limit point protection volume and, for the different azimuths covered, to model the trajectories of a compound maneuver beginning with change-of-heading maneuvers making it possible to reach the azimuth concerned and continuing with the standard vertical terrain avoidance maneuver, said trajectories being, in their parts corresponding to the standard vertical avoidance maneuver, predefined advantageously according to parameters such as the type of the aircraft, its weight and even its inertia or climb delay.